The present invention relates to a photoconductive layer of the electrophotographic photoconductors used in devices such as printers and copying machines that employ electrophotographic processes. More specifically, the present invention relates to constituent materials of the photoconductive layer.
The photosensitive materials of a conventional electrophotographic photoconductor (hereinafter simply referred to as a "photoconductor") used for devices such as printers, facsimiles, digital copying machines and analog copying machines, that employ electrophotographic processes, include inorganic photoconductive materials such as selenium and selenium alloys, inorganic photoconductive materials such as zinc oxide and cadmium oxide dispersed into a resin binder, organic photoconductive materials such as poly-N-vinylcarbazole and poly(vinyl anthracene), and other organic photoconductive materials such as phthalocyanine compounds and bisazo compounds dispersed into a resin binder or deposited by vacuum deposition.
It is required for the photoconductor to exhibit the functions for retaining surface charges in the dark, for generating electric charges in response to the received light, and for transporting the electric charges in response to the received light. The photoconductor may be classified into two types: 1) the mono-layered-type that exhibits the above described functions by one single photoconductive layer and 2) the so-called laminate-type consists of a layer mainly for charge generation, and a second layer for charge retention in the dark and charge transport in response to the received light.
For image formation by the electrophotographic techniques and with these types of photoconductors, Carlson's process can be applied as an example. The Carlson's process for image formation includes the steps of 1) charging of the photoconductor by corona discharge in the dark, 2) formation of electrostatic latent images of the letters and figures in a manuscript on the charged surface of the photoconductor, 3) development of the electrostatic latent images with toner, and 4) fixing of the developed toner images on a paper and such carriers. The photoconductor is reused after removal of the charge, removal of the residual toner, and removal of the optical charge.
Various image formation steps are employed in the Carlson's process. The corotron method or the scrotron method that uses metal wire and the contact charging method that uses the charging brush or charging roller are adopted for charging the photoconductor. Methods such as the two-components development method, nonmagnetic-single-component development method and magnetic-single-component development method are used in the development step.
Recently, the organic photoconductors have been developed by virtue of the flexibility, thermal stability and ease of film formation thereof. U.S. Pat. No. 3,484,237 discloses a photoconductor that includes poly-N-vinyl carbazole and 2,4,7-trinitrofluorenone. Japanese Unexamined Laid Open Patent Application No. S47-37543 discloses a photoconductor that includes an organic pigment as the main component thereof. Japanese Unexamined Laid Open Patent Application No. S47-10785 discloses a photoconductor that includes an eutectic complex consisting of a dye and resin as the main component thereof. At present, the function-separation-type organic photoconductors, which include a charge generation layer and a charge transport layer, are mainly used. The charge generation layer comprises metal-free phthalocyanine, metal phthalocyanine such as titanyl phthalocyanine or azo compound and a resin binder. The charge transport layer comprises a hydrazone compound, styryl compound, diamine compound or butadiene compound and a resin binder.
Among the function-separation-type photoconductors which laminate a charge generation layer on a conductive substrate and a charge transport layer on to the charge generation layer, negative-charging photoconductors exhibit sensitivity when the photoconductor surface is charged negatively, because the hole contributes to charge transport due to the nature of the charge transport material that functions as the electron donor. However, the corona discharge for negative-charging is unstable compared with the corona discharge for positive-charging. The corona discharge for negative-charging generates ozone and nitrogen oxide. The photoconductor surface is deteriorated physically and chemically by the ozone and nitrogen oxide absorbed thereto. Ozone and nitrogen oxide are very hazardous for the environmental safety. Accordingly, positive-charging photoconductors can be in practice more freely used and more widely used than negative-charging photoconductors.
Various positive-charging photoconductors have been proposed. Some positive-charging photoconductors, which include a single-layered photoconductive layer comprising a charge generation agent and a charge transport agent, both dispersed into a resin binder, have been put into practical use. However, the sensitivity of these positive-charging photoconductors of the single-layered type is not so high enough to be applicable to the high speed machines. More improvements are necessary for repeatedly using the positive-charging photoconductors of the single-layered type.
Laminate-type positive-charging photoconductors for high-speed use may be constructed by laminating a charge generation layer on a charge transport layer. However, corona discharge, light irradiation and mechanical wear pose problems of stability for repeated use of the photoconductor, since the charge generation layer is exposed on the surface of the photoconductor. The protection layer, disposed on the charge generation layer to avoid the mechanical wear of the charge generation layer, is problematic for improving the sensitivity and electrical properties of the photoconductor.
Laminate-type positive-charging photoconductors which include a charge transport layer on a charge generation layer have been proposed. The charge transport materials including 2,4,7-trinitrofluorenone may be used. However, 2,4,7-trinitro-9-fluorenone is a carcinogen. The Japanese Unexamined Laid Open Patent Applications No. S50-131941, No. H06-59483 and No. H06-123986 disclose cyano compounds and quinone compounds as the charge transport agent. Nonetheless, no charge transport agent that can be satisfactorily used for the laminate-type positive-charging photoconductor has yet been obtained.
Although the organic photoconductive materials have many merits which the inorganic photoconductive materials do not have, the conventional organic photoconductive materials do not exhibit all the properties required for electrophotographic photoconductors. It is required to fabricate a highly sensitive photoconductor that exhibits little change in the properties thereof after the photoconductor is continuously used in the electrophotographic apparatus continuous for a long time. Especially, the customer's demands are increasing for photoconductors, which can endure long continuous use in various electrophotographic apparatuses provided the foregoing with various imaging processes. The photosensitivity of the conventional laminate-type photoconductors is insufficient. Practical long use of the conventional laminate-type photoconductors causes charge potential lowering, residual potential rise, sensitivity lowering and such problems to be solved. Thus, a technology that facilitates realizing all the favorable properties for the electrophotographic photoconductor has not been established so far.